It is well known that micronised waxes will impart these desirable features to the paint or lacquer film. However, when a silica is coated/impregnated with the wax a process usually performed by jointly grinding the silica and wax in a fluid energy mill (microniser), further product benefits are immediately available. The mixed phenomenon of coating and impregnating, resulting from the inherent porosity of the silica, is hereinafter referred to as "coating". The wax can improve compatibility of the silica with the paint or lacquer preventing interaction with other components in the formulation, but another function is to prevent the formation of hard sediment during storage which is not redispersible. Whilst the mechanism by which this protection is afforded is still not fully understood, its benefit to the paint or lacquer manufacturer is widely recognised because, if hard sediments which cannot be redispersed are formed, none of the benefits described above can be realised.
1. Literature
Two types of wax coated silicas are generally disclosed in the literature. GB 798,621 (Grace) discloses a silica matting agent produced by co-milling an intermediate density silica gel with a microcrystalline wax in a fluid energy mill. GB 1,236,775 teaches a silica matting agent can be prepared by treating precipitated silica with an aqueous emulsion or dispersion of a wax, including thermoplastic materials. The properties of the silica matting agents prepared by the so-called "dry" co-micronising route have been further improved by adding fatty acids (GB 1,461,511 - Grace) or synthetic polyethylene waxes of varying molecular weight (U.S. Pat. No. 4,097,302--SCM) to the microcrystalline wax before feeding to the fluid energy mill. The former improves the dispersibility of the wax coated products, whereas the latter shows products having enhanced settlement characteristics compared with commercially available materials can be prepared. The "dry" processing routes described above use temperatures in excess of the melting points of the waxes or wax blends whereas GB 1,538,474 (BASF) discloses a process which can produce satisfactory wax coated silicas at micronising temperatures of below 50.degree. C., where functional waxes such as montan acid ester are employed.